Shell-and-tube transfer line heat exchangers are in widespread use in commercial chemical processing. In general, they operate to cool hot gases by passing these gases through a bundle of tubes in heat exchange relationship with a cooling fluid, such as water, passing around the outside, or shell side, of the tubes and contained within a defined area by means of a pair of tubesheets which are generally perpendicular to the tubes contained within them. In certain processes, the heat removed from the process gas is sufficient to vaporize the fluid on the shell side. In such cases if water is used as the cooling fluid the heat exchanger also becomes a steam generator.
TLEs are commonly used to cool very hot process gases. For example, they are used in processes for producing ammonia such as that disclosed in U.S. Pat. No. 3,442,613, issued May 6, 1969 to Grotz, to cool the approximately 850.degree. F. ammonia-containing gas exiting a syngas converter. They are also used in olefin plants and in other hydrocarbon cracking operations to recover usable heat from reactor gases, e.g., gases resulting from the cracking of ethane, propane, butane or mixtures thereof exiting pyrolysis furnace coils at temperatures above 1500.degree. F. To avoid secondary reactions leading to less valuable or useless products, the residence time spent by the exiting gases between the furnace coil outlet and the TLE inlet should be minimized. The pressure drop across the TLE should also be minimized, since cracking selectivity towards more useful products in the furnace is directly dependent on cracking-coil outlet pressure, and ordinarily a pressure rise of no more than a few p.s.i. at the furnace outlet is all that can be tolerated if process stability is to be maintained. A discussion of various TLE designs is found in Albright et al, "Pyrolysis Theory and Industrial Practice" (New York: Academic Press, 1983), Chapter 18.
The efficiency with which heat is recovered by a TLE can have a marked effect on plant operating costs. Inlet end fouling due to coke buildup can impair this efficiency to a substantial extent. At higher temperatures in processes where coking is a problem, very hard and often refractory layers of coke or carbon can form on the walls of the reactor, conduits and heat exchangers. This coke buildup will cause an increase in pressure drop across the TLE, which is detrimental to cracking yields and eventually requires a shutdown of the equipment to permit decoking.
It is difficult to examine in detail all of the reaction mechanisms occurring in chemical processes carried out at extremely high temperatures. Consequently, the mechanism(s) responsible for coke buildup in processes involving the use of TLEs have never been entirely elucidated. Some believe that it is important to keep the TLE tubes at a temperature above the dew point of any materials present which have a tendency to coke or deposit; see U.S. Pat. No. 4,405,440, issued Sept. 20, 1983 to Gwyn. Others believe it to be important to keep the connector between the reactor and the TLE at a temperature below 450.degree. C., well below that of the exiting gas stream, on the theory that if a gas stream, e.g., one flowing at a mass velocity below 50 kg/m.sup.2 per second, is quickly cooled to a temperature well below the temperature at the reactor exit, coking will not occur; see U.S. Pat. No. 4,151,217, issued Apr. 24, 1979 to Amano et al, and U.S. Pat. No. 4,384,160, issued May 17, 1983 to Skraba.
Other prior art methods of ameliorating the coking problem or attempting to prevent coking from occurring altogether have generally fallen into one of three categories:
prevention of coke buildup by means of substances added to the gas stream (see U.S. Pat. Nos. 3,174,924, issued Mar. 23, 1965 to Clark et al; 4,097,544, issued June 27, 1978 to Hengstebeck, and the Skraba patent, each of which discloses injecting a quench fluid or fluids into the gas stream being cooled) or added to the TLE tubes themselves (see U.S. Pat. Nos. 3,073,875, issued Jan. 15, 1963 to Braconier et al and 4,288,408, issued Sept. 8, 1981 to Guth et al, which disclose methods of forming a liquid or a gas film on the inner surfaces of the reactor, the tubes or both);
mechanical or chemical/mechanical means for cleaning out coke deposits once formed; see U.S. Pat. No. 4,248,834, issued Feb. 3, 1981 to Tokumitsu, which discloses decoking by feeding air through the system after interrupting the gas stream exiting the reactor, and U.S. Pat. No. 4,366,003, issued Dec. 28, 1982 to Korte et al, which discloses the use of high speed gas streams delivered from jet nozzles positioned above the TLE inlet openings to periodically flush the inlets clean, and
various mechanical modifications of the TLEs or surrounding equipment, such as the use of an inlet screen or sieve medium disposed in the inlet portion of the TLE at an easily accessible portion thereof to cause coke to build up on the screen or sieve; coke is removed by removing the used screen or sieve (U.S. Pat. No. 3,880,621, issued Apr. 29, 1975 to Schneider et al), varying tube size to equalize flow through each of the TLE tubes (U.S. Pat. No. 4,397,740, issued Aug. 9, 1983 to Koontz), "insulating" the tubes with heat transfer medium which is thinner at the inlet end and increases in thickness gradually or uniformly to a point at or near the end of the tubes (the Gwyn patent), using an expansion section and conduits to inject water to form a steam sheath adjacent to the walls of the expansion section (U.S. Pat. No. 3,574,781, issued Feb. 14, 1968 to Racine et al), using a precooler closely followed by a pair of aftercoolers connected in parallel (U.S. Pat. No. 3,607,153, issued Sept. 21, 1971 to Cijer), connecting a conically ended heat exchanger directly to a cracking heater outlet (U.S. Pat. No. 3,456,719, issued July 22, 1969 to Palchik), and using a bundle of triple tubes (U.S. Pat. No. 3,903,963, issued Sept. 9, 1975 to Fuki et al).
None of these expedients has fully served the intended purpose, and coking at TLE inlet ends remains a significant problem to the involved segments of the chemical processing industry.
One successful solution to this problem is disclosed and claimed in U.S. patent application entitled "Flow Streamlining Device For Transfer Line Heat Exchangers", Ser. No. 864,018, filed May 16, 1986 in the name of Carlton K. Shen-Tu and of common assignment with this application.
There has now been discovered another combination of expedients which minimizes or prevents entirely TLE inlet end fouling by coke buildup during high temperature chemical processing, and thus minimizes increased pressure drop across the system. This in turn optimizes heat recovery, process dynamics and process stability, and permits longer process runs between shutdowns.
It is, therefore, an object of this invention to provide novel transfer line heat exchangers.
Another object of this invention is to provide improved processes of quenching or recovering heat from high temperature fluids, and particularly high temperature gases, which involve the use of my novel transfer line heat exchangers.
A further object of this invention is to provide novel transfer line heat exchangers modified at their inlet ends by means which when employed together with a gas reactive with coke deposits, minimize or prevent inlet end fouling due to coke buildup.
These and other objects, as well as the nature, scope and utilization of this invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.